This invention relates to selective removal of COS from gas streams.
It is often necessary or desirable to reduce the COS content of gas streams (e.g., coke oven gas or geothermal steam) to levels compatible with further processing or use. Generally, however, only a small fraction of the COS can be removed by the absorbent solutions used for removing other sulfur-containing impurities. For example, H.sub.2 S can be removed by aqueous or water-containing solutions of organic bases or alkali salts of weak inorganic or organic acids, but COS is substantially unaffected by such solutions because it is chemically inert to the absorbent under conditions normally employed.
Physical scrubbing processes involving solubilization of impurities are also well known for treating acid gases. In general, COS has low solubility in the solvents used in such processes, and is not effectively removed.
One approach to the problem has been to convert COS to H.sub.2 S, which can then be removed in a conventional manner. Verloop et al. U.S. Pat. No. 4,153,674 (1979) describes one such process, which involves treating COS-containing gas with free hydrogen or free carbon monoxide at temperatures in the range of 180.degree. C. to 450.degree. C., in the presence of a catalyst consisting of a Group VI and/or Group VIII metal supported on an inorgainic oxide carrier, so as to convert most or all of the COS to H.sub.2 S and CO.sub.2.
Bratzler et al. U.S. Pat. No. 3,966,875 (1976) describes an alternative method by which, at 50.degree. C. to 105.degree. C., gas is contacted with a low volatility organic solvent inert to COS and containing 15 to 50 mole-percent water; the hydrolysis products (H.sub.2 S and CO.sub.2) and other sulfur-containing compounds are then removed by subsequent scrubbing of the gas.
Other process schemes involve hydrolyzing COS with an aqueous polyalkanolamine solution containing at least 20% by weight of tetramethylene sulfone (Sykes U.S. Pat. No. 3,965,244 (1976)) or using special amines, such as piperazinone, to catalyze hydrolysis (Bozzelli et al. U.S. Pat. No. 4,100,256 (1978)).
Generally, these absorption schemes are complex, particularly in conjunction with arrangements for regenerating the absorbing solutions. In addition, the mass action relation for hydrolysis of COS to H.sub.2 S and CO.sub.2 is displaced in the opposite (than desired) direction when the gas contains large quantities of carbon dioxide and hydrogen sulfide, causing intolerable concentrations of COS to persist in the treated gas. Furthermore, whenever COS is hydrolyzed to H.sub.2 S and CO.sub.2 by a basic solution, the CO.sub.2 content of the absorbent (and, eventually, of the desorbed gases from regeneration of the absorbent) is undesirably increased.
Other sulfur removal schemes are based on the difference in absorption rates between CO.sub.2 and H.sub.2 S. Because CO.sub.2 and COS have similar absorption properties, such schemes remove only H.sub.2 S, but are not effective in removing COS.
Harvey et al. U.S. Pat. No. 4,192,854 (1980) describes a process for removing H.sub.2 S from a gas stream involving contacting the stream with an ammonium sulfate-buffered CuSO.sub.4 solution, which precipitates out the sulfur in H.sub.2 S as copper sulfide. Nothing in the technical literature suggests that such a solution would be capable of reacting with COS.
While reduction of sulfur to low concentrations is required in most commercial applications, in a number of cases it is not necessary or desirable that carbon dioxide also be removed. For example, sour natural gas, which may contain several hundred ppm of COS in addition to substantial quantities of H.sub.2 S and CO.sub.2, must be desulfurized before it is used, but CO.sub.2 need not be removed to meet current pipeline specifications for natural gas. Also, in certain gasification processes, removal of CO.sub.2 is undesirable because it reduces the total volume of the gas available to generate power.
It is thus apparent that a process which selectively removes COS (and, if present, the other major form of sulfur, H.sub.2 S), but does not remove other acid gases, particularly CO.sub.2, would be highly advantageous. Even in cases where it is desirable to remove CO.sub.2, a selective process for COS removal would allow greater flexibility in the selection of a simultaneous or subsequent removal process for CO.sub.2, as well as in the achievement of a CO.sub.2 product stream of high purity.